Benedict XVI: “Now we know who God is!” Answer: He is the radical gift of Himself to the Father. He is nothing but pure relation to the Father. Therefore, in Himself, He is nothing. And that is what He is reduced to on the Cross. He is revealing that there is nothing in Him that is Him except relation to the Father.
J. Ratzinger - Colosseum 2005:
“In Greek and Latin, the two international languages of the time, and in Hebrew, the language of the Chosen People, a sign stood above the Cross of Jesus, indicating who he was: the King of the Jews, the promised Son of David. Pilate, the unjust judge, became a prophet despite himself. The kingship of Jesus was proclaimed before all the world. Jesus himself had not accepted the title "Messiah", because it would have suggested a mistaken, human idea of power and deliverance. Yet now the title can remain publicly displayed above the Crucified Christ. He is indeed the king of the world. Now he is truly "lifted up". In sinking to the depths he rose to the heights. Now he has radically fulfilled the commandment of love, he has completed the offering of himself, and in this way he is now the revelation of the true God, the God who is love. Now we know who God is. Now we know what true kingship is. Jesus prays Psalm 22, which begins with the words: "My God, my God, why have you forsaken me?" (Ps 22:2). He takes to himself the whole suffering people of Israel, all of suffering humanity, the drama of God's darkness, and he makes God present in the very place where he seems definitively vanquished and absent. The Cross of Jesus is a cosmic event. The world is darkened, when the Son of God is given up to death. The earth trembles. And on the Cross, the Church of the Gentiles is born. The Roman centurion understands this, and acknowledges Jesus as the Son of God. From the Cross he triumphs ever anew.”
Who, then, is God? He is the radical gift of Himself to the Father. He is nothing but pure relation to the Father. Therefore, in Himself, He is nothing. And that is what He is reduced to on the Cross. He is revealing that there is nothing in Him that is Him except relation to the Father. The name “Jesus Christ” means that He is Jesus, the Christ. His teaching is not His own. His work is not His own. His being is not His own. “The Son is Son, and in so far as He is Son, does not proceed in any way from himself and so is completely one with the Father; since he is nothing beside him, claims no special position of his own, confronts the Father with nothing belonging only to him, retains no room for his own individuality, therefore he is completely equal to the Father. The logic is compelling: if there is nothing in which he is just he, no kind of fenced-off private ground, then he coincides with the Father, is ‘one’ with him.” In a word, “substance” is not word that can describe the ontological reality that is the Son as the Person of Jesus Christ. And if Jesus Christ is the prototype of the human person, and the human person must find his ontological fulfillment in the Person of Christ, then, the human person cannot be described by “substance” either, where relation to the Father could be construed as “happening” to the person as “accident.”
Benedict XVI in “Jesus of Nazareth” II: “Recent theology has rightly underlined the use of the word ‘for’ in all four accounts, a word that may be considered the key not only to the Last Supper accounts, but to the figure of Jesus overall. His entire being is expressed by the word ‘pro-existence’ – he is there, not for himself, but for others. This is not merely a dimension of his existence, but its innermost essence and its entirety. His very being is a ‘being-for.’ If we are able to grasp this, then we have truly come close to the mystery of Jesus, and we have understood what discipleship is” (134).
 J. Ratzinger, “Introduction to Christianity,” Ignatius (1990) 134.